using System.Text.Json.Nodes;
using System.Text.RegularExpressions;

using MongoDB.Entities;

using PostEntity = PrivaPub.Models.Post.Post;

namespace PrivaPub.Federation.Inbox
{
	// Some servers name one of our posts by the address of its page instead of its id (Pixelfed likes /@name/<post id>, the
	// post's `url`). Before an activity is handled, such a reference to a post of ours, as the activity's object or as the
	// object of the activity it undoes, is replaced by the post's id, so that every handler finds the post the same way.
	public static partial class LocalReferences
	{
		[GeneratedRegex("^/@[^/?#]+/([0-9a-f]{24})$")]
		private static partial Regex PostPage();

		public static async Task Canonicalise(JsonNode activity, string baseAddress, CancellationToken token)
		{
			if (activity is not JsonObject outer || string.IsNullOrEmpty(baseAddress))
				return;
			await Replace(outer, baseAddress, token);
			if (outer["object"] is JsonObject inner)
				await Replace(inner, baseAddress, token);
		}

		static async Task Replace(JsonObject node, string baseAddress, CancellationToken token)
		{
			if (node["object"] is not JsonValue value || !value.TryGetValue<string>(out var uri) || PostId(uri, baseAddress) is not { } id)
				return;
			var post = await DB.Default.Find<PostEntity>().Match(p => p.ID == id && !p.IsFederatedCopy).ExecuteFirstAsync(token);
			if (post?.ObjectURI is { } canonical)
				node["object"] = canonical;
		}

		// the id of the post whose page the address is, on our own site
		public static string PostId(string uri, string baseAddress) =>
			uri.StartsWith(baseAddress + "/@", StringComparison.OrdinalIgnoreCase) && PostPage().Match(uri[baseAddress.Length..]) is { Success: true } page
				? page.Groups[1].Value
				: default;
	}
}